Displays and electronic devices have evolved to be curved, bent, or folded and provide new user experiences. These device architectures may include flexible organic light emitting diodes (OLEDs), plastic liquid crystal displays (LCDs) and the like, for example.
A typical electronic display includes a lower module, a display module, and an upper module. The lower module may contain heat spreaders and cushioning materials. The upper module may contain a touch sensor, an ambient light rejection layer such as a circular polarizer, and often a protective film or cover. These three components are often bonded together using a form of adhesion. When the bonded construction is flexed, strains develop throughout the structure. The strains are not uniform through the thickness and depend in a complex way on the design of the display. If strains exceed a critical threshold in the display module, the display module will mechanically fail, resulting in display defects, general malfunction, or both.
In any bending structure, there is a location through the thickness where there is zero strain known as the neutral bending plane. The location of the neutral plane can, in some cases, be controlled so that strains and stresses are minimized in the display module. If the moduli of the layers above and below the display are similar and the total thickness above and below the display are approximately equal, then by the symmetry of the stack the neutral plane is near the middle (i.e., near the display module). Also, the modulus and thickness of the adhesive placed proximate to the display module and which mechanically couples it to the upper or lower module may be selected to adjust the location of the neutral plane. If the adhesive is substantially more compliant than the adjacent layers, the layers may become partially mechanically decoupled in bending, resulting in multiple neutral planes. Multiple neutral planes may then be positioned in or near vulnerable display components to minimize the stress and strain they experience. In an alternative of the same approach, the compliant layer is chosen such that shear decoupling occurs at the interface between the display module and the layer above it so that the state of stress (compressive or tensile) in the display module is of an opposite sense to that in the upper module (i.e., tensile or compressive). However, the strategy of controlling neutral plane location or introducing multiple neutral planes in order to shield the display module is technically challenging, and slight variations in the material properties or thickness may shift the neutral plane above or below its intended location.
Thus, there is a need for an electronic display which can be folded or bent around a tight radius, for example over a wide range of strain rates or under consideration of other factors.